'Equivalent' 35 mm format (24x36 mm) focal lengths

By Q.G. de Bakker

A very frequently asked question is one about the 35 mm format focal length equivalent of the lenses used on
Hasselblad's 6x6 format.
It is often asked, perhaps because there is no single answer. The aspect ratios of 6x6 format (1:1) and
35 format (1:1.5) differ considerably, and an answer will depend on what aspect is used in the comparison.

Figure 1Same diagonal angle of view

Diagonal

Manufactureres usually only state the diagonal angle of view of the lenses they
produce. As a result, the diagonal often has to be taken as the basis for comparisons.

In Figure 1, 6x6 format (black outline) and 35 mm format (red outline) frames that have the same size diagonal
are superimposed.

As Figure 1 shows, the different aspect ratios of both formats lead to a rather different framing: the 6x6
frame is taller than the 35 mm format frame, yet less wide.
A subject just fitting inside the 35 mm frame horizontally will not fit inside the 6x6 frame, and conversely, a
subject just fitting inside the 6x6 frame vertically will not fit inside the 35 mm frame.

Framing in both formats is equivalent only in the rare case that subjects have to be framed diagonally.
The differences in both horizontal and vertical aspects then still make it very hard, if not impossible, to
crop an diagonally composed image in either format to the image produced by the other format.

Figure 2Same vertical angle of view

Vertical

Another option would be to select lenses so that their vertical angles of view
are the same (Figure 2).
The difference in aspect ratios of the two formats then shows itself in the horizontal angle of view, which is
1.5 times as large on 35 mm format as that of the equivalent lens on 6x6 format.
It obviously is very easy to crop the larger frame to produce an image exactly the same as that produced inside
the smaller frame. The (vertical) equivalence is true.

Figure 3Same horizontal angle of view

Horizontal

Conversely, when the lenses are chosen so that the horizontal angles of view are
the same (Figure 3), the difference in aspect ratio leads to a 1.5 times larger vertical angle of view on 6x6 format.
Again, cropping the taller frame to produce an image exactly the same as that captured in the smaller frame is
not a problem, i.e. the (horizontal) equivalence is true.

The Hasselblad focal lengths given in the table to the right are nominal. The actual focal lengths differ,
not only from this nominal focal length, but also from that of other Zeiss/Hasselblad lenses having nominally
the same focal length.

To keep the table simple, the choice was made to use the nominal focal lengths instead of the true focal
lengths.
As a result the table only gives an approximation of the equivalent 35 mm format focal lengths.

Since the known focal lengths of 35 mm format lenses are also nominal, with their true focal lengths rarely known,
an approximation will be the best a general comparison can achieve. For critical purposes, an actual test of all
the lenses involved will be necessary.

6x6 format lens

35 mm format lens having the

same horizontal angle of view

same vertical angle of view

same diagonal angle of view

38 mm

24 mm

16 mm

21 mm

40 mm

25 mm

17 mm

22 mm

50 mm

32 mm

21 mm

27 mm

60 mm

38 mm

25 mm

32 mm

80 mm

51 mm

34 mm

43 mm

100 mm

64 mm

42 mm

54 mm

105 mm

67 mm

45 mm

57 mm

110 mm

70 mm

47 mm

60 mm

120 mm

76 mm

51 mm

65 mm

135 mm

86 mm

57 mm

73 mm

150 mm

96 mm

64 mm

81 mm

180 mm

115 mm

76 mm

97 mm

250 mm

159 mm

106 mm

135 mm

300 mm

191 mm

127 mm

162 mm

350 mm

223 mm

149 mm

190 mm

500 mm

319 mm

212 mm

271 mm

Figure 4Same horizontal angle of view

Figure 5Same vertical angle of view

6 x 4.5 format

Ever since Hasselblad introduced a film magazine producing 16 frames in 6x4.5
format in 1956, 6x6 is not the only Hasselblad film format.

The difference in the aspect ratios of 35 mm format and 6x4.5 format is small. The 6x4.5 format is a little
bit taller than the 35 mm format. Consequently, the spread in equivalent focal lengths depending on what
aspect is chosen to be equal is also smaller.

Figures 4 and 5 show the difference in framing when lenses are chosen so that either the horizontal or
vertical angles of view are equal (blue outline = 6x4.5 format).

6x4.5 format lens

35 mm format lens having the

same horizontal angle of view

same vertical angle of view

same diagonal angle of view

38 mm

24 mm

22 mm

23 mm

40 mm

25 mm

23 mm

25 mm

50 mm

32 mm

29 mm

31 mm

60 mm

38 mm

34 mm

37 mm

80 mm

51 mm

46 mm

49 mm

100 mm

64 mm

57 mm

61 mm

105 mm

67 mm

60 mm

65 mm

110 mm

70 mm

63 mm

68 mm

120 mm

76 mm

69 mm

74 mm

135 mm

86 mm

77 mm

83 mm

150 mm

96 mm

86 mm

92 mm

180 mm

115 mm

103 mm

111 mm

250 mm

159 mm

143 mm

154 mm

300 mm

191 mm

171 mm

184 mm

350 mm

223 mm

200 mm

215 mm

500 mm

319 mm

286 mm

307 mm

The Hasselblad focal lengths given in the table to the left are nominal. The actual focal lengths differ,
not only from this nominal focal length, but also from that of other Zeiss/Hasselblad lenses having nominally
the same focal length.

To keep the table simple, the choice was made to use the nominal focal lengths instead of the true focal
lengths.
As a result the table only gives an approximation of the equivalent 35 mm format focal lengths.

Since the known focal lengths of 35 mm format lenses are also nominal, with their true focal lengths rarely known,
an approximation will be the best a general comparison can achieve. For critical purposes, an actual test of all
the lenses involved will be necessary.

4 x 4 'superslide' format

The third and last Hasselblad format to consider is the 4x4 cm 'superslide' format.
It was introduced in 1957, to produce slides that could be projected using normal 35 mm format projectors.

The aspect ratio is the same as that of the original 6x6 format, so the same considerations apply. Though
being much smaller than the 6x6 format, the angles of view will be smaller too. The equivalent 35
mm format focal lengths will be longer accordingly.

The Hasselblad focal lengths given in the table to the right are nominal. The actual focal lengths differ,
not only from this nominal focal length, but also from that of other Zeiss/Hasselblad lenses having nominally
the same focal length.

To keep the table simple, the choice was made to use the nominal focal lengths instead of the true focal
lengths.
As a result the table only gives an approximation of the equivalent 35 mm format focal lengths.

Since the known focal lengths of 35 mm format lenses are also nominal, with their true focal lengths rarely known,
an approximation will be the best a general comparison can achieve. For critical purposes, an actual test of all
the lenses involved will be necessary.